1. Field of the Invention
The present invention relates to a method for operating a gas turbine group.
2. Brief Description of the Related Art
The stability of combustion in modern premix burners of gas turbines is critically dependent in part on the ignitability of the fuel used, but also on other fuel properties, such as in particular the calorific value or the Wobbe index.
Premix burners of this type have been disclosed, for example, by EP 321 809, EP 780 629, WO 93/17279 or WO 92/19913. The various designs of burner are based on the common idea of introducing fuel into a swirling combustion-airstream and of generating an as far as possible homogenous and usually substoichiometric, lean fuel-air mix. When it is transferred into the combustion space, the swirling flow bursts open at a change in cross section, with the result that a backflow region which serves for flame stabilization is formed upstream of the burner mouth. The flame must not be stabilized too close to the burner mouth, in order to prevent excessive thermal loading of the burner. However, if the stabilization zone is too far downstream of the burner mouth, instability results.
The position of the combustion zone is also decisively dependent on the ignitability of the fuel used. This changes dramatically if, for example, a fuel gas contains high levels of higher saturated hydrocarbons, such as ethane, butane, propane, also known as C2+alkanes. Since the fuel-air mix is supplied in premixed form, there is an acute risk of flashback to the burner. Component failure is then likely.
A similar problem also manifests itself when operating spontaneous-ignition combustion chambers of the type that are known from EP 669 500, for example in a gas turbine group with sequential combustion, as has been disclosed by EP 620 362. In this case too, flashback caused by a highly ignitable fuel gas can lead to major damage.
Furthermore, a change in the fuel-gas composition also has effects on the emissions and on combustion-chamber pulsations.
In addition to the C2+-alkane content, the calorific value or Wobbe index also has an influence on the combustion behavior. In premix burner systems, this influence is primarily attributable to the changed pulse ratio of fuel gas and air at different Wobbe numbers. U.S. Pat. No. 6,082,092 describes controlling a variable fuel-gas preheating in such a way that the Wobbe index is kept constant. Control with a view to establishing a constant Wobbe index would also be possible by variable admixing of inert components in a closed-loop control circuit. DE 197 31 209 proposes keeping the Wobbe index constant when burning residual gases of highly varying composition by the controlled admixing of natural gas and nitrogen.